The present invention provides rubber mixtures which contain a diene rubber with a concentration of primary hydroxyl groups of 0.1 to 2 wt. % and a glass transition temperature of xe2x88x92120 to xe2x88x9250xc2x0 C. and their mixtures with fillers, optionally other rubbers and rubber auxiliary substances and vulcanisates prepared therefrom. Rubber mixtures according to the invention are suitable for producing highly reinforced, abrasion-resistant moulded items, in particular for producing tires which have particularly high wet skidding resistance.
Double bond-containing solution rubbers, such as solution polybutadiene and solution styrene/butadiene rubbers, have advantages over the corresponding emulsion rubbers when producing low rolling resistance tire treads. The advantages are based, inter alia, on the ability to control the vinyl content, cis content and glass transition temperature and molecular branching associated therewith. Particular advantages in relation to abrasion, wet skidding resistance and rolling resistance of the tires result therefrom in a practical application. Thus, U.S. Pat. No. 5,227,425 describes the production of tire treads from a solution SBR rubber and silica. To further improve the properties, numerous methods for modifying the end groups have been developed, as is described e.g. in EP-A 334 042, with dimethylaminopropyl-acrylamide or, as described in EP-A 447,066, with silyl ethers. However, due to the high molecular weight of the rubber, the proportion by weight of end groups is small and can therefore have only a small effect on the interaction between filler and rubber molecule. The present invention is intended to provide solution diene rubbers such as solution polybutadiene and polyisoprene with a much higher concentration of effective groups for interacting with fillers.
A process for preparing hydroxyl group-containing solution polybutadiene rubbers is also described in DE-OS 2,653,144. These rubbers, however, have a much higher concentration of hydroxyl groups, associated with much higher glass transition temperatures.
EP-A 464,478 describes a process for hydroxylating rubber, wherein, however, secondary hydroxyl groups are introduced which are far less effective than the primary hydroxyl groups in the present invention.
Hydroxyl group-containing emulsion and solution rubbers are also described in EP-805,452 A1, wherein the hydroxyl concentrations described here for solution rubbers lie within a much lower range (0.009 to 0.061%), depending on the process used, and the glass transition temperatures are substantially higher ( greater than xe2x88x9240xc2x0 C.), depending on the styrene content, for the emulsion rubbers described.
It has now been found that rubber mixtures and rubber vulcanisates with surprisingly improved dynamic damping characteristics in the temperature range relevant to wet skidding resistance and the temperature range relevant to rolling resistance and also improved abrasion behaviour can be prepared from hydroxyl group-containing solution diene rubbers with a concentration of 0.1 to 2 wt. % of bonded primary hydroxyl groups and a glass transition temperature of xe2x88x92120 to xe2x88x9250xc2x0 C. Further surprising advantages were obtained when the rubber mixture was prepared not in conventional compounders but by mixing a solution of hydroxyl group-containing rubber and oxidic or siliceous fillers in organic solvents and the solvent was then removed using steam, since then the filler is fully precipitated with the rubber and does not remain in the waste water, as when using an unmodified rubber.